MOSCOW -- Russia opened a criminal case Tuesday against Greenpeace activists, accusing them of piracy, a charge that could carry a prison term of five to 15 years, authorities said.

Russian border troops seized a Greenpeace ice-breaker, the Arctic Sunrise, and its multinational crew of 30 activists and sailors, in a dramatic commando operation in the Barents Sea on Thursday, the day after the group attempted to raise a protest banner on a Russian oil drilling platform.

The ship was towed by the Russian coast guard to an anchor in Kola Bay, about six miles from the port of Murmansk.

“After conducting a preliminary investigation, the Russian Investigative Committee's northwestern branch initiated a criminal case on the signs of … piracy committed by an organized group,” Vladimir Markin, the investigative committee spokesman, said in a statement published on the agency's official website Tuesday.

No formal charges have been filed.

Markin seemed to suggest that the activists were masking a darker motive for boarding the oil platform.

He said that the Greenpeace vessel was crammed with “electronics of unclear purpose," adding that "when a group of persons claiming to be members of an ecological rights organization are trying to all but storm a drilling platform, it evokes reasonable doubts about their intent.”

Greenpeace has said the group intended to raise a banner on the Prirazlomnaya drilling platform to protest Arctic pollution. Prirazlomnaya is a major Arctic oil exploration project of Gazpromneft, a subsidiary of the Russian state-owned energy giant Gazprom.

Greenpeace spokeswoman Maria Favorskaya insisted that Russian authorities had acted illegally in seizing and towing the group's ship, which was traveling under the Netherlands flag. She added that authorities effectively turned the boat into an illegal prison.

"At first they accused our activists of terrorism, then of illegal scientific research activities and now they come up with the absurdist charge of them all -- piracy!" Favorskaya said in a telephone interview from Murmansk. "How can peaceful activists who simply tried to put up a poster up the side of an oil drilling platform be accused of such a serious felony?"

She said Greenpeace lawyers and diplomats from other countries have not been allowed to board the Arctic Sunrise.

"If any act of piracy took place in that area of the Arctic it was certainly committed by Russian border troops who illegally attacked our vessel and continue to hold our activists and the crew hostages, denying any access to them for a fifth day," Favorskaya said.

Markin, the investigative committee spokesman, said he found it hard to believe that "so-called activists didn't know that the platform is an object of increased risk and any unsanctioned actions on it can lead to an accident which would endanger not only the people onboard but the ecology" that Greenpeace "so zealously protected."

He said the protesters could have threatened "the ecological safety of the entire region."

Denis Sinyakov, a prominent Moscow-based photographer who was onboard the Arctic Sunrise, described last week's confrontation with authorities in dramatic detail on his Facebook page. The Greenpeace activists had approached the drilling platform in small boats that were launched from the Arctic Sunrise.

"The men in coast guard boats seemed to have lost self control," he wrote in Russian. "Screaming and pointing arms at the activists, they started to shoot first into the water. One soldier lost control, took out a knife and started to wave it in front of the main guy in our boat."

He said that two activists, whom he identified as Marco Polo of Switzerland and Sini Saarela of Finland, started to climb ropes that had been attached to the drilling platform but fell into the icy water after authorities cut the ropes. “The border troops made several holes in Greenpeace boats with their knives and after intensive shooting made the boats turn away,” he wrote.

Sinyakov made the entry on Wednesday. On Thursday, he was arrested along with the rest of the Arctic Sunrise crew.